Because getting the F/LOSS alternative to feature-parity is going to require a multi-million-dollar investment, and using the proprietary product is $30 / month?
This really isn't rocket science. You're not going to get tens of thousands of people to throw $10 / month at random borderline-MVP F/LOSS projects in the hope that they'll eventually mature - especially when there isn't even anything resembling a mature project management structure around it. Without serious corporate backing, most of those projects will go absolutely nowhere.